Design Considerations for DOI-encoding PET Detectors Using Phosphor-Coated Crystals

Driven by a need for high and uniform spatial resolution, as well as depth correction to improve time-of-flight PET, the design of PET detectors with depth-of-interaction (DOI) information is of great interest. To combine the advantages of continuous DOI information with a single-ended readout of the crystal, we have proposed a solution based on the use of phosphor-coated crystals. Here we build on our initial work to optimize the coating process and detector design, thus taking a significant step towards making this approach practical. We implemented a new method to coat 20 mm-long unpolished LSO crystals with a mixture of YAG phosphor powder and UV-curable optical cement that provides accurate, robust, and reproducible coatings. Using this new coating process, critical parameters such as coating thickness, coating shape, and crystal size were studied. The performance of different coating configurations was evaluated by recording pulses at different irradiation depths and extracting the decay time of individual pulses. The delayed charge integration method was used to characterize the pulses acquired at various DOI. The coating thickness was decreased from 250 μm (previous work) to 50 μm with minimal degradation in performance. 100 μm-thick coating provided the best trade-off between performance and additional volume around the crystal, which will be important to build crystal arrays with high packing fraction. Additionally, varying the coating length on the different crystal faces allowed for better DOI encoding along the crystal length, when compared to previous coating configurations. Crystal face sizes ranging from 2 mm × 2 mm to 5 mm × 5 mm were studied; best performance was obtained with the 2 mm × 2 mm crystals.